1. Field of the Invention
This application relates generally to the manufacture of hydrogen fluoride and, more specifically, to a process in which an improved fluorspar feed is provided for hydrogen fluoride manufacture.
2. Description of the Prior Art
It has long been known to produce hydrogen fluoride by reacting calcium fluoride with sulfuric acid in an externally heated rotary furnace. Fluorspar ore is generally employed as the source of the calcium fluoride. However, as naturally occurring fluorspar is generally found combined with other minerals such as calcite (a mineral containing calcium carbonate) and quartzite (a mineral containing silicon dioxide), it is desirable to remove the impurity-minerals to prevent consumption of sulfuric acid in the furnace, which occurs in reactions with calcium carbonate and silicon dioxide, illustrated by the following equations: EQU CaCO.sub.3 +H.sub.2 SO.sub.4 CaSO.sub.4 +CO.sub.2 +H.sub.2 O (1) EQU siO.sub.2 +2CaF.sub.2 +2H.sub.2 SO.sub.4 SiF.sub.4 +2CaSO.sub.4 +2H.sub.2 O (2)
the CO.sub.2 becomes an impurity in the HF gases produced, and increases problems associated with HF recovery.
One method employed to remove calcite and quartzite impurities from the fluorspar ore involves use of flotation techniques in which the ore is processed by grinding it to a desired degree of fineness, to liberate particles of calcite and quartzite, slurrying the ground ore with water, admixing the slurry with suitable flotation agent and creating a froth from the resulting admixture, as by blowing air through the aqueous ore slurry containing the flotation agent. The flotation agent coats the fluorspar particles, and allows these particles to be collected on the surface as part of the froth. At the same time, most of the liberated calcite and quartzite particles are caused to sink to the bottom of the vessel containing the froth and are discarded. This process results in ore containing as much as 97% by weight or more of calcium fluoride. The froth is removed from the flotation tank vessel, generally by allowing it to overflow the vessel, and is then treated (e.g., by filtration) to recover the concentrated fluorspar solids, which are subsequently dried before being passed for use in hydrogen fluoride manufacture.
While the above flotation techniques have resulted in concentrated fluorspar ore containing less silicon dioxide and calcium carbonate impurities, the presence of residual flotation agent on the dried concentrated fluorspar results in the formation of a foam in the furnace due to the generation in the reaction furnace of HF and carbon dioxide gases, the latter being caused by reaction of any residual calcium carbonate impurities with sulfuric acid.
The foam is highly undesirable. Not only does it reflect loss of raw material as a result of reaction of sulfuric acid and calcium carbonate therewith, but also the foam can cause blockage of gas lines, necessitating complete shutdown of the furnace and substantially reducing unit production. A substantial decrease in heat transfer to the reacting mass from the furnace walls is also caused by the foam, increasing energy requirements. While the volume of foam can be controlled by reducing feed to the furnace, this also reduces productivity of the unit.
This problem has been recognized by the prior art. Note, for example, U.S. Pat. No. 3,878,294 (issued Apr. 15, 1975 to W. Schabacher et al.) which seeks to avoid the problem by heating the fluorspar to a temperature of from 500.degree. to 800.degree. C. using a counterflow of hot gas in order to vaporize any residual flotation agent. However, such a process is undesirable due to the high energy consumption.
The theoretical relationship between certain solids and liquids and the creation in such solid/liquid systems of stabilized foams has been discussed in the literature. See, e.g., J.J. Bikermann, "Surface Chemistry Theory and Applications", p. 378 (2nd ed., Academic Press) (1958) and S. Ross, Chemical Engineering Progress, Vol. 63, No. 9, pg. 41 (1967). It is known that when an imperfectly wetted solid is contacted by a gas bubble the solid is drawn into the interface between the gas and liquid phases, and imparts rigidity to the thin liquid film of the bubble, preventing liquid from draining through the foam layer and stabilizing the foam. Increasing the available surface area of the solid phase increases the foam stability. Thus, the Bikermann reference discloses that coarse galena powder (particle diameter near 0.03 cm.) raised the duration of foam from 17 seconds to 60 seconds and fine galena (particle diameter near 0.01 cm.) to as much as several hours. Bikermann, supra, at p. 378.
However, the art has not investigated the effect particle sizes have upon the stabilization of foams during reaction of calcium flouride ores and sulfuric acid to form hydrogen fluoride.